The use of pressurized gas vessels, or air tanks, for breathable primary or supplemental air supplies has become widely accepted in recreational, industrial and public safety arenas. Air tanks are used underwater for recreational underwater diving, termed “SCUBA” (Self Contained Underwater Breathing Apparatus) diving, which is also applicable to industrial or commercial settings for ship repair, off-shore drilling operations, underwater salvage, pipeline repair, as well as to military applications, search and rescue operations, and underground environments, such as mines. In above-water applications, an air tank used as a Self-Contained Breathing Apparatus (SCBA) is invaluable for personnel working in an environment that poses an Immediate Danger to Life or Health (IDLH). SCBAs are used by fire fighters entering into smoke filled or toxic environments, police working at contaminated crime scenes, underground mine rescue teams entering “bad air” or smoke, HAZMAT teams working in contaminated environments, and industrial maintenance personnel working in confined spaces or in toxic environments. SCBAs are also strategically placed in chemical plants, laboratories, refineries, nuclear facilities, paper mills and underground mines for employees to use as a “self-rescuer” during an emergency.
FIG. 1 depicts a conventional pressure vessel or air tank 10 configured as a cylinder 12 with a domed, or hemispherical, top 18 and a domed, or hemispherical, bottom 20. A rubber or plastic boot 16 disposed over the bottom 20 both protects and stabilizes the cylinder 12 when resting on a surface in an upright position. A gas under pressure such as a breathable air mixture or, less commonly, oxygen, is injected and expelled from the cylinder 12 via a primary valve 14.
Cylinders vary in size depending upon the application. A self-rescue air cylinder may be relatively small; for example a cylinder with an 8 cubic foot equivalent capacity at 3000 PSI, may be 4 inches in diameter and 10 inches long, and can be used to supply sustainable air for about 5 minutes. This provides the user enough time to retreat from a small building or enclosure to a safe environment. An air supply lasting 15 minutes is the minimum requirement for an SCBA to be approved by the National Institute for Occupational Health and Safety (NIOSH) for entering IDLH environments. In order for a SCBA to meet the 15 minute standard the device must have a capacity of 20-24 cubic feet equivalent, when the air is pressurized to 3000 PSI. This requires a single air cylinder with dimensions approximately 5 inches in diameter and 18 inches in length, or the breathable air volume can be split between multiple, smaller cylinders. However, the bulk and size of either of these configurations may become a hindrance if the user is attempting to perform tasks in, or trying to escape from, a confined space. Fire fighters and mine rescue personnel use incrementally larger configurations of SCBAs, allowing for extended forays into IDLH environments, re-supplying breathable air to fellow rescuers or the ability to share breathable air with a victim stranded in the hostile environment. Most SCUBA air cylinders are a so-called standard “80,” or a cylinder capable of holding 80 cubic feet of air. Thus, if an additional volume of breathable air is required, most divers will use two tanks or “double-up” tanks. However, 100 cubic foot and larger tanks are also available. The 100 cubic foot tanks are extremely large and, when made from steel, weigh more than 40 lbs. Double tanks or the large, 100 cubic foot tanks are manageable underwater, but can significantly reduce the diver's mobility when negotiating around or within structures such as, by way of example only, rocks or coral, offshore platforms, or sunken ships. Additionally, smaller users may not be physically able to shoulder or handle the larger tanks or multi-tank configurations while above water.
Further, with respect to the use of multiple cylindrical tanks, stacking or aligning cylindrical structures in a single row is an inefficient use of space. Considering cylinders with the same diameter, each cylinder contacts the laterally adjacent cylinder or cylinders tangentially along a line of contact, creating a substantially triangular void between the cylinders on either side of the line of contact. This issue has been addressed in a number of applications for improving the volumetric efficiency of multiple cylinders used for storing propane or natural gas. Most of these multi-cylinder pressure vessels are formed using rolled steel sections which are mated and welded together, such as those described in U.S. Pat. No. 4,946,056 to Stannard, U.S. Pat. No. 3,528,582 to Rigollot and U.S. Pat. No. 3,414,153 to Leroux. Another method used to form multi-cylinder pressure vessels includes interlocking sections, such as a clip and lobe arrangement, shown in U.S. Pat. No. 6,220,779 to Warner et al. U.S. Pat. No. 5,944,215 to Orlowski describes a volumetrically efficient multi-cylinder vessel that may be formed from a plastic as a one-piece or unitary structure. A second one-piece plastic structure designed for use in automotive applications as a vacuum pressure vessel is described in U.S. Pat. No. 4,343,409 to Silver. Each of these plastic vessels are designed for relatively low pressure or vacuum use, the former being designed for at least 5 atmospheres or approximately 75 psi and the latter being described to provide “adequate implosion resistance” when subjected to a vacuum sufficient to actuate automotive features like headlight door covers.
A diving tank is typically attached to a pack-frame using a clamping ring, or the tank is placed into a “clamshell” structure that fully encases the tank and is worn on the diver's back. Originally the pack-frame accommodated the air tank only and the user would need a separate weight-belt and inflatable vest or BC (buoyancy compensator) to achieve neutral or slightly negative buoyancy while in the water. Modern tank packs are now universally referred to as a BC and are configured more like a vest which includes the air tank clamping ring, integrated regulators and gauges, an inflatable bladder, weights and pockets for numerous diving accessories. With both the older pack-frame and a modern BC, the air tank extends well above the diver's back and, since it is behind the diver and out of sight, the diver can easily misjudge the clearance necessary to enter an opening in a reef, cave or wreck, causing damage to the air tank or possibly trapping the diver. This problem is exacerbated when using large or multiple tanks.
Divers manage underwater risk in several ways. One way is by “slinging” their air tank over a single shoulder when diving in a confined space such as wreck or cave, allowing the tank to dangle below the diver, where the tank can be watched and manually manipulated around obstacles. A second way to manage risk and which has been adopted as a modern standard safety feature is the provision of an emergency regulator. Originally, tanks included a single hose from the primary valve to the diver's mouthpiece or regulator. Currently, an additional hose for a backup regulator or “octopus” is attached to the tank. The octopus is available to the diver if the primary regulator fails or malfunctions and may be offered to another diver who is short of air or otherwise in trouble.
It would be desirable to offer a portable air tank system that is volumetrically more efficient than conventional multi-tank systems, is of reduced overall weight for easy handling, is less obtrusive, as well as more compact and hydrodynamic.